Two effects can primarily be observed during imaging by ionizing rays, especially by X-rays, which occur as the radiation passes through the material, namely absorption and phase shifting of the radiation passing through an object. It is also known that in many cases, when a ray passes through an object under examination, the phase shift depends significantly more greatly on small differences with regard to the thickness and the composition of the penetrated material than the absorption. Basically the extent of the two effects depends in each case on the energy of the radiation, the density and the atomic number of the irradiated material.
The passage of X-rays through material can be described by a complex refractive index. The non-disappearing imaginary part of the refractive index describes the strength of the absorption of the material. The non-disappearing real part of the refractive index leads to a phase shift of the X-ray wave passing through the material. The object of phase-contrast X-ray imaging is, in addition to images of the absorption strength expressed by the linear coefficient of attenuation or the imaginary part of the refractive index of the object irradiated by the X-rays, to also measure images of the local phase or of the local gradients of the phase of the wave front running through the irradiated object.
In this case, in a similar way to X-ray radiography or X-ray tomography, both projectional images of the phase shift and also tomographic representations of the phase shift can be computed on the basis of a plurality of projectional images.
The phase of an X-ray wave cannot be directly determined in such cases but only by interference with a reference wave. The phase shifts relative to the reference waves or to the adjacent rays respectively can be measured by using interferometric gratings and be combined into projectional and/or tomographic recordings.
A structure of an X-ray tomography device which is suitable for carrying out phase-contrast measurements is known from EP 1 731 099 A1. In this structure a total of three gratings are needed. A first grating G0, which is also referred to as the source grating, is placed between the focal point of the X-ray tube and the position of the object. The second grating G1, which is referred to as the diffraction grating, phase grating or also Talbot grating, is placed between the object and the X-ray detector. The third grating G2 is referred to as the absorption grating and is arranged between the diffraction grating G1 and the X-ray detector.
The source grating G0 serves to provide a plurality of line sources which behave spatially partly coherent in relation to each other. The diffraction grating G1 serves to impress a spatial phase shift pattern on the incident wave front and thus to create an interference pattern behind the diffraction grating G1. At specific distances from this grating G1, at the Talbot distances, on arrival of a smooth wave at the grating G1, a more or less sharp interference pattern is produced. The absorption grating G2 serves to sample the interference pattern. The gratings G0, G1 and G2 are aligned in parallel to each other in such cases.
The detector is structured for locally-resolved detection of the electrical signals generated by conversion of incident radiation quantas in the form of pixels. The recording of an absorption image and of a phase-contrast image or of a differential phase-contrast image is undertaken in the known manner by measuring the intensity in each pixel of the detector as a function of the relative position of the gratings G0, G1 and G2 in relation to each other. In the known case for example the absorption grating G2 is shifted step-by-step or continuously perpendicular to the radiation direction and perpendicular to the slot direction. The intensity of the X-ray radiation is registered for each pixel in the form of an electrical signal as a function of the grating position. A modulated signal is involved here, which in the respective pixel represents the strength of the absorption in the ray path through the object to the pixel from the illuminated part of the grating G2. From the position of the maxima and minima of the intensity curve in the pixel, which depends on the projected gradient of the real part of the refractive index, the local phase shift of the X-rays can be computed. An integration of these gradients along a line perpendicular to the radiation direction and perpendicular to the slot direction delivers the local average phase of the wave front after its passage through the object projected onto the pixel.
The disadvantage of the known method is that in the absorption grating G2 radiation quantas which have passed the object will be absorbed and will thus no be longer available for imaging. This especially increases the dose necessary for imaging or the patient dose. A further disadvantage can be seen in the fact that changes in the geometry of the arrangement during the measurement of the intensity, for example with a change to a relative position of the gratings in relation to one another lead, through mechanical deformations as a result of centrifugal forces, as arise in CT applications, to a distorted modulation curve. This leads to an incorrect determination of the phase.